An evanescent-mode cavity filter is a resonant cavity that can be capacitively loaded by including a conical post in the center of the cavity. The post forms a parallel-plate capacitance in a small gap between the top of the post and the ceiling of the cavity. The ceiling of the cavity can be a movable structure, such as a flexible piezoelectric or MEMS electrostatically actuated membrane, so as to change the loading capacitance, thus allowing for tuning of the center frequency of the resonant cavity. Further details about tunable, evanescent-mode cavity filters may be found, for example, in U.S. Pat. No. 9,024,709 to Joshi et al., the entirety of which is hereby incorporated by reference.
There is a need to increase the responsiveness, accuracy, and stability of a series of tunable filters that cover a frequency range from approximately 1 to 110 GHz, while greatly reducing C-SWAP (Critical-Size, Weight and Power) and improving the filter performance beyond that of existing switched fixed filter systems.